Abstract:

Provided is a magnetic carrier giving a high quality image free of density
variation without the occurrence of fogging or carrier adhesion and
having excellent dot reproducibility even during long-term use. The
magnetic carrier has magnetic carrier particles produced by filling pores
of porous magnetic core particles with a resin. The magnetic carrier
contains 80% by number or more of the magnetic carrier particles
satisfying the specific conditions (a) and (b) when 18 straight lines
passing through a reference point of a cross section of the magnetic
carrier particle are drawn at intervals of 10° in a reflected
electron image of the cross section of the magnetic carrier particle
photographed by a scanning electron microscope.

Claims:

1. A magnetic carrier having magnetic carrier particles produced by
filling pores of porous magnetic core particles with a resin,wherein the
magnetic carrier contains 80% by number or more of the magnetic carrier
particles satisfying the following (a) and (b) when 18 straight lines
passing through a reference point of a cross section of the magnetic
carrier particle are drawn at intervals of 10.degree. in a reflected
electron image of the cross section of the magnetic carrier particle
photographed by a scanning electron microscope:(a) the number of magnetic
core regions having a length of 6.0 μm or longer on the straight lines
is from 5.0% by number or more to 35.0% by number or less relative to the
number of magnetic core regions having a length of 0.1 μm or longer on
the straight lines, and(b) the number of regions other than the magnetic
core part having a length of 4.0 μm or longer on the straight lines is
from 1.0% by number or more to 15.0% by number or less relative to the
number of regions other than the magnetic core part having a length of
0.1 μm or longer on the straight lines.

2. The magnetic carrier according to claim 1, wherein the ratio of the
area of the magnetic core region to the area of the cross section of the
magnetic carrier particle is from 50% by area or more to 90% by area or
less in the reflected electron image of the cross section of the magnetic
carrier particle photographed by the scanning electron microscope.

3. The magnetic carrier according to claim 1, wherein the magnetic carrier
particles are particles where a surface of the particles produced by
filling pores of porous magnetic core particles with a resin is further
coated with a resin.

4. A two component developer comprising at least a magnetic carrier and a
toner, wherein the magnetic carrier is a magnetic carrier according to
claim 1.

5. The two component developer according to claim 4, wherein in the toner,
the content of the particle having a diameter of 4.0 μm or less on a
number basis is 35.0% by number or less and the content of the particle
having a diameter of 12.7 μm or more on a volume basis is 3.0% by
volume or less.

Description:

[0001]This application is a continuation of International Application No.
PCT/JP2009/064092, filed on Aug. 4, 2009, which claims the benefit of
Japanese Patent Application No. 2008-200644 filed on Aug. 4, 2008.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a magnetic carrier and a two
component developer used for an electrophotographic method, an
electrostatic recording method, and an electrostatic printing method.

[0004]2. Description of the Related Art

[0005]For example, a ferrite carrier containing a heavy metal has
conventionally been used as a carrier. However, such a carrier has a high
density and further a large saturated magnetization, and thus a magnetic
brush becomes so stiff that deterioration of a developer, such as carrier
spent and deterioration of an external additive for toner, can take place
easily.

[0006]Accordingly, in order to lower specific gravity, a carrier having a
surface having very small asperities and an inner structure having many
fine voids is proposed (refer to Japanese Patent Application Laid-Open
No. H08-050377). The above-mentioned carrier maintains the chargeability
because a carrier surface is always ground down in a development unit
thereby exposing a newly formed surface. However, the thus ground down
carriers increase in the developer during long-term use thereby
decreasing the fluidity of the developer and this, in turn, causes
density variation (a decrease in image uniformity) and fogging in some
cases.

[0007]A resin-filled ferrite carrier produced by filling voids of the
ferrite having a porosity of 10 to 60% and an intercommunicating porosity
of 1.8 to 4.0 with a resin is proposed (refer to Japanese Patent
Application Laid-Open No. 2006-337579). Although the above-mentioned
carrier has a lower specific gravity, a higher durability is obtained by
controlling a void structure. However, a local difference in the charged
electric amount occurs on a carrier surface after toner development,
thereby causing a density variation and lowering a dot reproducibility in
some cases, and thus there has been room for improvement in such a
carrier.

[0008]Accordingly, a carrier having a sterically laminated structure in
which a resin layer and a ferrite layer are present alternately is
proposed (refer to Japanese Patent Application Laid-Open No.
2007-057943). The above-mentioned carrier has a stable chargeability by
properties like a capacitor. However, the laminated structure is so dense
that filling the void part present near to the center of a core material
with a resin is prone to be insufficient. As a result, there has been a
case that part of the magnetic carrier was destroyed during long-term
durability use, leading to carrier adhesion. Furthermore, the carrier is
excessively charged due to the presence of voids, and thus the need still
exists to obtain a high quality image stably.

SUMMARY OF THE INVENTION

[0009]An object of the present invention is to provide a magnetic carrier
and a two component developer which are free from the problems as
mentioned above. Specifically, an object of the present invention is to
provide a magnetic carrier and a two component developer giving a high
quality image free of density variation without the occurrence of fogging
or carrier adhesion and having excellent dot reproducibility even during
long-term use.

[0010]The present invention relates to a magnetic carrier having magnetic
carrier particles produced by filling pores of porous magnetic core
particles with a resin, characterized in that the magnetic carrier
contains 80% or more by number of the magnetic carrier particles
satisfying the following (a) and (b) when 18 straight lines passing
through a reference point of a cross section of the magnetic carrier
particle are drawn at intervals of 10° in a reflected electron
image of a cross section of the magnetic carrier particle photographed by
a scanning electron microscope; (a) the number of magnetic core regions
having a length of 6.0 μm or longer on the straight lines is from 5.0
to 35.0% by number (inclusive) relative to the number of magnetic core
regions having a length of 0.1 μm or longer on the straight lines, and
(b) the number of regions other than the magnetic core part having a
length of 4.0 μm or longer on the straight lines is from 1.0 to 15.0%
by number (inclusive) relative to the number of regions other than the
magnetic core part having a length of 0.1 μm or longer on the straight
lines.

[0011]Further, the present invention relates to a two component developer
containing a magnetic carrier and a toner, characterized in that the
magnetic carrier is the magnetic carrier mentioned above.

[0012]By using the magnetic carrier of the present invention, a highly
precise and fine image can be formed stably. Specifically, a high quality
image free of density variation without the occurrence of fogging or
carrier adhesion and having excellent dot reproducibility even during
long-term use can be obtained.

[0013]Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view of a surface modifying apparatus.

[0015]FIG. 2 is one example of a cross section of the magnetic core
particle of the present invention.

[0016]FIG. 3 is one example of a reflected electron image by SEM
designating only the processed cross section region of the magnetic
carrier particle of the present invention.

[0017]FIG. 4 is a schematic view of one measurement example of the
magnetic core region and regions other than the magnetic core part in a
cross section of the magnetic carrier particle of the present invention.

[0018]FIG. 5 is one example showing distribution of the length and the
numbers (% by number) obtained by measuring the magnetic core region
having a length of 0.1 μm or longer and the regions other than the
magnetic core part having a length of 0.1 μm or longer in the cross
section of the magnetic carrier particle of the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0019]When a toner is developed, a counter electric charge having a
polarity opposite to that of the toner remains inside a magnetic carrier.
This part having a built-up counter electric charge has a high adhesion
strength with the toner, which will not leave from the magnetic carrier
particles readily. Accordingly, the charging sites on a surface of the
magnetic carrier particles decrease, resulting in a large decrease in
chargeability as the magnetic carrier. In addition, the toner developed
on an electrostatic image carrier is pulled back to a developer carrier
by the counter electric charge, resulting in deterioration of the
developing properties of the toner.

[0020]In order to prevent this phenomenon, the counter electric charge of
the magnetic carrier needs to be drained to the developer carrier
smoothly through the magnetic carrier. By doing so, the power to pull
back the toner as mentioned above is eliminated, and thus excellent
developing properties can be obtained.

[0021]However, if a magnetic carrier having a core particle having low
resistance was merely used in order to drain the counter electric charge,
in some cases, an electrostatic latent image on an electrostatic image
carrier and a toner image are disturbed. This is because the resistance
of the magnetic carrier is so low that a leakage between the
electrostatic image carrier and the developer carrier occurs via chain
formation of the magnetic carrier formed on the developer carrier, and
this in turn leads to disturbance of the electrostatic latent image and
the toner image. In order to improve the developing properties without
disturbing the electrostatic latent image, control of carrier's electric
properties in such a manner as to drain the counter electric charge to
the developer carrier without the leakage between the developer carrier
and the electrostatic image carrier is important.

[0022]In view of the above, the present inventors have found that, in the
magnetic carrier particles produced by filling pores of the porous
magnetic core with a resin, the above-mentioned problems could be solved
by controlling the existence state of the magnetic core part and the
resin part inside the particles. Specifically, the magnetic carrier
having magnetic carrier particles produced by filling pores of porous
magnetic core particles with a resin need to satisfy the following.
Namely, on the 18 straight lines drawn at intervals of 10° which
pass through a reference point of a cross section of the magnetic carrier
particle in a reflected electron image of the cross section of the
magnetic carrier particle photographed by a scanning electron microscope,
the number of magnetic core regions having a length of 6.0 μm or
longer is from 5.0 to 35.0% by number (inclusive) relative to the total
number of the magnetic core region having a length of 0.1 μm or
longer, and the number of regions other than the magnetic core part
having a length of 4.0 μm or longer is from 1.0 to 15.0% by number
(inclusive) relative to the total number of the region other than the
magnetic core part having a length of 0.1 μm or longer. By controlling
the inner structure of the magnetic carrier in such a manner as mentioned
above, the magnetic carrier having excellent developing properties
without disturbance of the electrostatic latent image due to the leakage
as mentioned above can be obtained. Though the detailed reason for this
is not clear yet, inventors of the present invention assume the following
for it.

[0023]At the time of image formation, a plurality of magnetic carrier
particles form a chain in the state of a point-to-point contact on the
developer carrier. Especially in the developing region where the toner is
developed to the electrostatic image carrier, the magnetic carrier
particles line up on a nearly straight line along a magnetic force line.
At this time, each magnetic carrier particle comes in contact with its
adjacent magnetic carrier particles at two points (poles). A straight
line connecting the contact points (a straight line connecting the two
poles) is a diameter of the magnetic carrier particle. Usually, an
electric charge moves on the diameter line, which is the shortest path.

[0024]Here, a porous magnetic core particle is a bound body of grains
(sintered primary particle) obtained by sintering various fine particles
at high temperature. The bound body of the grains corresponds to the
magnetic core region of the magnetic carrier particle. The state of the
body greatly affects strength and electric properties as the carrier. The
above-mentioned counter electric charge moves via the magnetic core
region inside the magnetic carrier particle. In the case of the porous
magnetic core particle which has been proposed so far, the contacting
area of grains is small because the grains are small, and thus adhesion
among grains is low. Accordingly, an electric charge among grains cannot
move smoothly, thereby the counter electric charge resides inside the
carrier, resulting in pull back of a toner, which in turn causes
difficulty in toner development in some cases.

[0025]To solve this problem, it is necessary to make the transfer of the
electric charge among grains smooth by making grains relatively large in
the porous magnetic core particle and controlling the binding in such a
way as to secure a large contacting area among grains.

[0026]As a result of investigation based on the above-mentioned finding,
it was found that the smooth transfer of the counter electric charge
among grains and the excellent developing properties could be obtained by
controlling, on 18 straight lines passing through a reference point of a
cross section of the magnetic carrier particle drawn at intervals of
10°, the number of magnetic core regions having a length of 6.0
μm or longer from 5.0 to 35.0% by number (inclusive). More
advantageously, the number of magnetic core regions having a length of
6.0 μm or longer on the straight lines is from 10.0 to 30.0% by number
(inclusive). In addition, it is advantageous that the magnetic core
region longer than 25.0 μm do not exist.

[0027]When the number of magnetic core regions having a length of 6.0
μm or longer is less than 5.0% by number, the counter electric charge
with a reverse polarity to the toner which remains inside a magnetic
carrier cannot be drained smoothly from the magnetic carrier surface,
resulting in difficult toner development. When the number of magnetic
core regions having a length of 6.0 μm or longer is more than 35.0% by
number, the leakage of an electric charge via chain formation of the
magnetic carriers tends to occur easily.

[0028]On the other hand, in order to prevent leakage of an electric charge
between the electrostatic image carrier and the developer carrier via
chain formation of the magnetic carrier formed on the developer carrier,
the existence state of "the region other than the magnetic core part" is
important. Namely, the region other than the magnetic core part
corresponds to pores of the porous magnetic core particle, and in the
present invention a resin is filled in most of this region. An electric
charge does not move via a resin basically, and thus the leakage is more
difficult to occur with a larger ratio of the pores in the porous
magnetic core particle. Accordingly, to define the existence state of the
region other than the magnetic core part in a cross section of the
carrier particle is important.

[0029]Accordingly, in the carrier particle of the present invention, the
number of regions other than the magnetic core part having a length of
4.0 μm or longer on the 18 straight lines drawn at intervals of
10° which pass through a reference point of a cross section of the
magnetic carrier particle is from 1.0 to 15.0% by number (inclusive).
More advantageously, the number of regions other than the magnetic core
part having a length of 4.0 μm or longer is from 2.0 to 10.0% by
number (inclusive). In addition, it is advantageous that the region other
than the magnetic core region having a length of longer than 12.0 μm
do not exist.

[0030]When the number of regions other than the magnetic core part having
a length of 4.0 μm or longer is within the above-mentioned range, the
leakage of an electric charge between the electrostatic image carrier and
the developer carrier can be prevented even under the flow of the counter
electric charge.

[0031]When the length of the region other than the magnetic core part is
less than 4.0 μm, the distance between the magnetic core regions is
small and an electric current flows also in the region other than the
magnetic core part because the developing region is in a high electric
field, and thus suppression of the leakage becomes difficult. As a
result, a flow of the electric charge cannot be controlled sufficiently.

[0032]When the number of regions other than the magnetic core part having
a length of 4.0 μm or longer is less than 1.0% by number, the leakage
of an electric charge between the electrostatic image carrier and the
developer carrier via chain formation of the carrier occurs readily,
thereby disturbing an electrostatic latent image and a toner image in
some cases. In addition, because pores of the porous magnetic core
particle cannot contain a resin sufficiently, a physical strength of the
magnetic carrier particle decreases. As a result, a part of the magnetic
carrier is destroyed during long-term durability use, which leads to the
carrier adhesion and the fogging due to decrease in the chargeability in
some cases.

[0033]When the number of regions other than the magnetic core part having
a length of 4.0 μm or longer is more than 15.0% by number, difference
in specific gravity within magnetic carrier particles increases thereby
decreases in fluidity of the magnetic carrier, resulting in the image
variation in some cases. Further, the carrier is excessively charged
electrically, resulting in decrease in developing properties in some
cases.

[0034]As mentioned above, in order to suppress the leakage of an electric
charge between the developer carrier and the electrostatic image carrier
while draining the counter electric charge to the developer carrier, it
is important that the relationship between the magnetic core region and
the region other than the magnetic core part in the cross section of the
carrier particle satisfy the range defined by the present invention.

[0035]In the magnetic carrier of the present invention, the total number
of the magnetic core region having a length of 0.1 μm or longer on the
18 straight lines drawn at intervals of 10° which pass through a
reference point of a cross section of the magnetic carrier particle is
advantageously from 50 to 250 (inclusive), and more advantageously from
70 to 200 (inclusive). In addition, the total number of the region other
than the magnetic core part having a length of 0.1 μm or longer on the
above-mentioned straight lines is advantageously from 50 to 250
(inclusive), and more advantageously from 70 to 200 (inclusive). When the
total number of each region is within the above-mentioned range, a
filling amount of a resin into pores of the porous magnetic core particle
can be easily controlled, and thus the flow of an electric charge inside
the magnetic carrier can be controlled more easily.

[0036]In addition, it is necessary that, in the magnetic carrier of the
present invention, the ratio of the magnetic carrier particles satisfying
the range of the percentage by number of the magnetic core region having
a length of 6.0 μm or longer and the percentage by number of the
region other than the magnetic core part having a length of 4.0 μm or
longer, as defined above, be 80% or more by number relative to the total
carrier particles. Further, the ratio of the above-mentioned magnetic
carrier particles is more advantageously 92% or more by number.

[0037]In the magnetic carrier particle of the present invention, the ratio
of the area of the magnetic core region to the total area of the cross
section of the magnetic carrier particle is advantageously from 50 to 90%
by area (inclusive) in a reflected electron image photographed by a
scanning electron microscope.

[0038]When the area ratio of the magnetic core region of the magnetic
carrier is made within the above-mentioned range, a specific gravity of
the magnetic carrier can be controlled small, and in addition physical
strength can be secured satisfactorily. As a result, miscibility with the
toner is improved further and at the same time the stress on the carrier
at the time of mixing can be reduced, and thus stable image quality can
be secured for long.

[0039]The magnetic carrier particles of the present invention are
advantageously particles where a surface of the particles produced by
filling pores of porous magnetic core particles with a resin is further
coated with a resin. By coating the surface of the particle filled with a
resin further with a resin, an environmental stability improves further.
Especially even under an environment of a high temperature and a high
humidity, the thus coated carrier is excellent against fogging and change
in the image density caused by decrease in the charged electric amount.

[0040]The porous magnetic core particle has very small asperities on its
surface formed by crystal growth in formation of the particle. These
asperities also affect the surface character of the magnetic carrier
particle after a resin is filled, resulting in a minute difference in the
chargeability by friction between a depressed portion and a raised
portion in some cases. Especially when the particle is left under the
environment of a high temperature and a high humidity, the electric
amount charged by friction in the toner decreases readily. When an image
was generated under this state, there was a case that the change in image
density was large. Accordingly, by coating the surface of the particle
having a filled resin further with a resin, the difference due to
asperities is decreased, and thus the problem as mentioned above can be
remedied.

[0041]Furthermore, in the magnetic carrier of the present invention, the
area ratio of the void part region not filled with the resin is
advantageously 15% or less by area and more advantageously 10% or less by
area relative to the total area of the cross section of the magnetic
carrier particle in a reflected electron image photographed by a scanning
electron microscope.

[0042]When the area ratio of the void part region not filled with the
resin in the magnetic carrier is within the above-mentioned range, pores
of the porous magnetic core particle are filled with the resin
satisfactorily, and thus the magnetic carrier is excellent in physical
strength and not destroyed readily even under a stress during long-term
durability use. Furthermore, the above-mentioned range is also
advantageous in order to control the flow of an electric charge inside
the magnetic carrier particle as mentioned above.

[0043]Then, the porous magnetic core will be described. In the present
invention, the term "porous magnetic core" means an aggregate of a number
of porous magnetic core particles. It is important that the porous
magnetic core particles have pores connecting from the surface of the
magnetic core particle to its inside. The magnetic carrier can have an
increased strength and excellent developing properties by filling the
pores with a resin.

[0044]Materials for the porous magnetic core particle are advantageously a
magnetite or a ferrite, while a ferrite is more advantageous.

[0045]The ferrite is a sintered body represented by the following formula:

(M12O)x(M2O)y(Fe2O3)z

wherein, M1 represents a monovalent metal, M2 represents a divalent metal,
and when x+y+z=1.0, x and y are respectively 0≦(x, y)≦0.8,
and z is 0.2<z<1.0.

[0046]In the above formula, M1 and M2 are advantageously one or more metal
atoms selected from the group consisting of Li, Fe, Mn, Mg, Sr, Cu, Zn,
Ni, Co, and Ca. Specific examples thereof are metal compounds including
magnetic Li ferrites such as (Li2O)a(Fe2O3)b
(0.0<a<0.4, 0.6≦b<1.0, and a+b=1) and
(Li2O)a(SrO)bFe2O3)c (0.0<a<0.4,
0.0<b<0.2, 0.4≦c<1.0, and a+b+c=1); Mn ferrites such as
(MnO)a(Fe2O3)b (0.0<a<0.5, 0.5≦b<1.0,
and a+b=1); Mn--Mg ferrites such as
(MnO)a(MgO)b(Fe2O3)c (0.0<a<0.5,
0.0<b<0.5, 0.5≦c<1.0, and a+b+c=1.0); Mn--Mg--Sr ferrites
such as (MnO)a(MgO)b(SrO)c(Fe2O3)d
(0.0<a<0.5, 0.0<b<0.5, 0.0<c<0.5, 0.5≦d<1.0,
and a+b+c+d=1); and Cu--Zn ferrites such as
(CuO)a(ZnO)b(Fe2O3)c (0.0<a<0.5,
0.0<b<0.5, 0.5≦c<1.0, and a+b+c=1). The above-mentioned
ferrites may contain a minute amount of other metals.

[0047]In order to make a porous structure and a state of asperities on the
core surface suitable, Mn-containing ferrites, namely, Mn ferrites,
Mn--Mg ferrites, and Mn--Mg--Sr ferrites, are more advantageous in view
of easy control of the growth rate of the ferrite crystal and appropriate
control of the specific resistance of the porous magnetic core.

[0048]In the following, the manufacturing steps when a ferrite is used as
the porous magnetic core will be described in detail.

[0049]Step 1 (Step of Weighing and Mixing):

[0050]Weighed amounts of ferrite raw materials are taken into a mixing
apparatus, and then crushed and mixed for a time ranging from 0.1 hour to
20.0 hours (inclusive). Examples of the ferrite raw materials include Li,
Fe, Zn, Ni, Mn, Mg, Co, Cu, Ba, Sr, Y, Ca, Si, V, Bi, In, Ta, Zr, B, Mo,
Na, Sn, Ti, Cr, μl, a metal particle of a rare earth metal, an oxide
of a metal element, a hydroxide of a metal element, an oxalic acid salt
of a metal element, and a carbonate salt of a metal element.

[0051]The mixing apparatus includes a ball mill, a planetary mill, a
giotto mill, and a vibration mill. Especially, a ball mill is
advantageous in view of mixing performance.

[0052]Step 2 (Step of Tentative Calcination):

[0053]The ferrite raw material mixture is tentatively calcined in an air
at a calcination temperature ranging from 700° C. to 1,000°
C. (inclusive) and with the time ranging form 0.5 hours to 5.0 hours
(inclusive) to make a ferrite from the raw materials. For calcination, a
burner-type calcination furnace, a rotary-type calcination furnace, or an
electric furnace is used, for example.

[0054]Step 3 (Step of Crushing):

[0055]Tentatively calcined ferrite obtained in Step 2 is crushed by a
crushing machine.

[0056]There is no restriction in the crushing machine as far as a desired
particle diameter can be obtained.

[0057]Examples of the crushing machine include a crusher, a hammer mill, a
ball mill, a bead mill, a planetary mill, and a giotto mill.

[0058]The 50% particle diameter on a volume basis (D50) of a pulverized
product of the tentatively calcined ferrite is advantageously from 0.5
μm to 5.0 μm (inclusive), and the 90% particle diameter on a volume
basis (D90) is advantageously from 2.0 μm to 7.0 μm (inclusive).
Further, D90/D50, the indicator of the particle size distribution of the
pulverized product of the tentatively calcined ferrite, is advantageously
from 1.5 to 10.0 (inclusive). With these, the percentage by number of the
magnetic core region and the percentage by number of the region other
than the magnetic core part can be readily controlled within the range
defined in the present invention.

[0059]In order to obtain the pulverized product of the tentatively
calcined ferrite having the above-mentioned particle diameter, in the
case of a ball mill and a bead mill for example, it is advantageous to
select a material for a ball and a bead and to control an operation time.
Specifically, in order to obtain the tentatively calcined ferrite having
a smaller particle diameter, a ball with a higher specific gravity may be
selected, or a crushing time may be made longer. Furthermore, in order to
control the particle size distribution of the pulverized product of the
tentatively calcined ferrite within the above-mentioned range, mixing a
plurality of tentatively calcined ferrites having different particle
diameters is advantageous.

[0060]The material for the ball and the bead is not particularly
restricted as far as an intended particle diameter and a distribution can
be obtained. Examples thereof include glasses such as soda glass
(specific gravity of 2.5 g/cm3), sodaless glass (specific gravity of
2.6 g/cm3), and soda glass with a high specific gravity (specific
gravity of 2.7 g/cm3); quartz (specific gravity of 2.2 g/cm3);
titania (specific gravity of 3.9 g/cm3); silicon nitride (specific
gravity of 3.2 g/cm3); alumina (specific gravity of 3.6 g/cm3);
zirconia (specific gravity of 6.0 g/cm3); steel (specific gravity of
7.9 g/cm3); and stainless steel (specific gravity of 8.0
g/cm3). Among them, alumina, zirconia, and stainless steel are
advantageous in view of good abrasion resistance.

[0061]The size of the ball and the bead is not particularly restricted as
far as intended particle diameter and distribution can be obtained. For
example, the ball with a diameter from 5 mm to 60 mm (inclusive) is
suitably used, and the bead with a diameter from 0.03 mm to 5 mm
(inclusive) is suitably used. In a ball mill and in a bead mill, the wet
type shows a higher crushing efficiency than the dry type, because the
crushed product is not stirred up in the mill. Accordingly, the wet type
is advantageous to the dry type.

[0062]Step 4 (Step of Granulation):

[0063]Pulverized product of the tentatively calcined ferrite may be added
by a dispersing agent, water, a binder, and, as appropriate, a pore
controlling agent.

[0065]In the case that the crushing in Step 3 is done with the wet type,
in light of water contained in the ferrite slurry, it is advantageous to
add a binder and, as appropriate, a pore controlling agent.

[0066]The thus obtained ferrite slurry is dried and granulated by an
atomization drier at a heating temperature ranging from 100° C. to
200° C. (inclusive). There is no particular restriction in the
atomization drier as far as an intended particle diameter of the porous
magnetic core is obtained. For example, a spray drier may be used.

[0067]Step 5 (Step of Main Calcination):

[0068]Then, a granulated product is calcined at the temperature ranging
from 800° C. to 1,300° C. (inclusive) and with the time
ranging from 1 hour to 24 hours (inclusive). The temperature ranging from
1,000° C. to 1,200° C. (inclusive) is more advantageous. By
making the time for raising the temperature shorter and the time for
lowering the temperature longer, the rate of crystal growth can be
controlled to obtain an intended porous structure. The holding time of
the calcination temperature is advantageously from 3 hours to 5 hours
(inclusive) in order to obtain an intended porous structure. In order to
obtain the area ratio ranging from 50 to 90% by area (inclusive) in the
magnetic core region of the cross section of the magnetic carrier
particle, it is advantageous to control the calcination temperature and
the calcination time within the above-mentioned ranges. Calcination of
the porous magnetic core is facilitated by raising the calcination
temperature or making the calcination time longer, thereby resulting in a
larger area ratio of the magnetic core region.

[0069]Step 6 (Step of Classification):

[0070]After the calcined particles are parted as mentioned above, coarse
particles or fine particles may be removed, as appropriate, by sieving
them with a classifier or a sieve machine.

[0071]Here, the 50% particle diameter on a volume basis (D50) is
advantageously from 18.0 μm to 58.0 μm (inclusive) in view of
improved chargeability by friction to the toner and suppression of the
fogging and the carrier adhesion to the image.

[0072]The porous magnetic core obtained in the way as mentioned above is
prone to a poor physical strength and thus readily breakable depending on
the number or the size of the pore. Because of this, the carrier particle
of the present invention is filled with a resin in pores of the porous
magnetic core particle.

[0073]The method for filling a resin into pores of the above-mentioned
porous core particle is not particularly restricted. The method in which
a resin solution obtained by mixing a resin and a solvent is penetrated
into pores of the porous magnetic core particle and then the solvent is
removed is advantageous. In the case that the resin is soluble in an
organic solvent, the organic solvent such as toluene, xylene, cellosolve
butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, and methanol
may be used. In the case that the resin is water-soluble or of an
emulsion type, water may be used as the solvent.

[0074]The amount of the resin as the solid content in the above-mentioned
resin solution is advantageously from 1 to 30% by mass (inclusive), and
more advantageously from 5 to 20% by mass (inclusive). When the resin
solution with the resin amount of more than 30% by mass is used, the
resin solution cannot readily penetrate into pores of the porous magnetic
core particle uniformly because of a high viscosity. When the amount is
less than 1% by mass, the resin amount is so small that removal of the
solvent takes longer time, resulting in nonuniform filling or poor
adhesion strength of the resin to the porous magnetic core particle in
some cases.

[0075]The resin used to fill the pores of the above-mentioned porous
magnetic core particle is not particularly restricted. Any of a
thermoplastic resin and a thermosetting resin may be used, and the one
having a high affinity for the porous magnetic core is advantageous. When
the resin having a high affinity is used, a surface of the resin-filled
magnetic carrier can be coated readily by a resin after pores of the
porous magnetic core particle are filled by the resin.

[0077]Examples of the above-mentioned thermosetting resin include a phenol
resin, a modified phenol resin, a malein resin, an alkyd resin, an epoxy
resin, an unsaturated polyester (obtained by polycondensation of maleic
anhydride, terephthalic acid, and a polyalcohol), an urea resin, a
melamine resin, an urea-melamine resin, a xylene resin, a toluene resin,
a guanamine resin, a melamine guanamine resin, an acetoguanamine resin, a
glyptal resin, a furane resin, a silicone resin, a polyimide, a polyamide
imide resin, a polyether imide resin, and a polyurethane resin.

[0078]These resins may also be modified for use. Among them,
polyvinylidene fluoride resin, a fluorocarbon resin, a fluorinated resin
such as a perfluorocarbon resin or a solvent-soluble perfluorocarbon
resin, and a modified silicone resin or a silicone resin are advantageous
because of their high affinity for the porous magnetic core particles.

[0079]Among the above-mentioned resins, a silicone resin is particularly
advantageous. A silicone resin heretofore known may be used as the
silicone resin.

[0081]The amount of the resin to be filled in pores of the porous magnetic
core particle is advantageously from 5.0 to 25.0 parts by mass
(inclusive) relative to 100 parts by mass of the porous magnetic core in
view of controllability of the ease of the leakage inside the magnetic
carrier particle. More advantageous amount is from 8.0 to 20.0 parts by
mass (inclusive).

[0082]It is advantageous that the magnetic carrier of the present
invention be used after pores of the porous magnetic core particle are
filled with a resin and then further its surface is coated with a resin
in the light of the control of a releasability, an anti-fouling property,
a chargeability by friction, a resistance of the magnetic carrier, and
the like. In this case, the resin used for filling and the resin used as
a coating material for coating may be the same or different, and a
thermoplastic resin or a thermosetting resin.

[0083]The resin to form the above-mentioned coating material is
exemplified by the above-mentioned thermoplastic resins and the
above-mentioned thermosetting resins. Modified resins of these resins may
also be used. Examples thereof include a fluorinated resin such as a
polyvinylidene fluoride resin, a fluorocarbon resin, a perfluorocarbon
resin or a solvent-soluble perfluorocarbon resin, and a modified silicone
resin.

[0085]The above-mentioned resins may be used singly or as a mixture of
them. In addition, a thermosetting resin may be used by mixing with a
curing agent and the like and being cured. Especially, a resin having a
further higher releasability is suitably used.

[0086]The amount of the resin for coating the surface of the porous
magnetic core particle filled with a resin is advantageously from 0.1 to
3.0 parts by mass (inclusive) relative to 100 parts by mass of the porous
magnetic core particle filled with a resin. The amount is more
advantageously from 0.3 to 2.0 parts by mass (inclusive). When the
coating amount is made within the above-mentioned range, the
chargeability by friction of the magnetic carrier and an environmental
stability can be improved.

[0087]In addition, a conductive particle or a charge controlling particle
may be mixed for use as the coating resin. Examples of the conductive
particle include carbon black, magnetite, graphite, zinc oxide, and tin
oxide. The amount to be added is advantageously from 0.1 to 10.0 parts by
mass (inclusive) relative to 100 parts by mass of the coating material in
view of control of the resistance of the magnetic carrier.

[0088]Examples of the charge controlling particle include an
organometallic complex particle, an organometallic salt particle, a
chelate compound particle, a monoazo metal complex particle, an
acetylacetone metal complex particle, a hydroxycarboxylic acid metal
complex particle, a polycarboxylic acid metal complex particle, a polyol
metal complex particle, a polymethyl methacrylate resin particle, a
polystyrene resin particle, a melamine resin particle, phenol resin
particle, a nylon resin particle, a silica particle, a titanium oxide
particle, and an alumina particle. The amount of the charge controlling
particles to be added is advantageously from 0.5 to 50.0 parts by mass
(inclusive) relative to 100 parts by mass of the coating resin in view of
control of the amount of the electric amount charged by friction.
Especially there may be mentioned the following charge controlling
materials to be used in a silicone resin.

[0090]The method for further coating a surface of the magnetic carrier
filled with a resin with the resin after pores of the porous magnetic
core particle are filled with a resin is not particularly restricted.
Examples of the applying method to be used for the coating include a
dipping method, a spray method, a brush coating method, and a fluidized
bed method.

[0091]The 50% particle diameter on a volume distribution basis (D50) of
the magnetic carrier of the present invention is advantageously from 20.0
μm to 60.0 μm (inclusive). The above-mentioned specific range is
advantageous in view of the chargeability by friction to the toner and
the suppression of the carrier adhesion and the fogging. Here, the 50%
particle diameter (D50) of the magnetic carrier may be controlled by a
wind classification and a sieve classification.

[0092]Then, the toner contained along with the magnetic carrier in the two
component developer of the present invention will be described. In the
toner used in the present invention, the content of the particle having a
diameter of 4.0 μm or less on a number basis is advantageously 35.0%
or less by number and the content of the particle having a diameter of
12.7 μm or more on a volume basis is advantageously 3.0% or less by
volume in order to obtain both high quality image and durability. When
the particle size distribution of the toner is within the above-mentioned
range, fluidity of the toner is excellent, sufficient charged electric
amount can be readily obtained, and fogging can be suppressed easily.

[0093]Further, the weight-average particle diameter (D4) of the toner is
advantageously from 4.5 μm to 10.0 μm (inclusive), and more
advantageously from 5.0 μm to 9.0 μm (inclusive). When the
weight-average particle diameter (D4) of the toner is within the
above-mentioned range, the dot reproducibility improves further.

[0094]The average circularity of the toner used in the present invention
is advantageously from 0.940 to 1.000 (inclusive). When the average
circularity of the toner is within the above-mentioned range, the
releasability of the carrier and the toner is excellent. Here, the
average circularity is based on the circularity distribution of the
circle equivalent diameter ranging from 1.985 μm to 39.69 μm
(inclusive), wherein the circularity measured by a flow-type particle
image measurement apparatus with an image processing resolution power of
512×512 pixels (0.37 μm×0.37 μm per one pixel) in one
visual field is divided into 800 in the range from 0.200 to 1.000
(inclusive) of the circularity for analysis.

[0095]When the toner with the average circularity being within the
above-mentioned range is used with the magnetic carrier of the present
invention, fluidity as the developer can be appropriately controlled. As
a result, transportation properties of the two component developer on the
developer carrier become excellent and the toner can be released readily
from the magnetic carrier, and thus the toner can be developed more
easily.

[0096]A binding resin having the following properties is advantageous in
order to satisfy both storage stability and low-temperature fixing
properties. Namely, the peak molecular weight (Mp) of the molecular
weight distribution is from 2,000 to 50,000 (inclusive), the
number-average molecular weight (Mn) is from 1,500 to 30,000 (inclusive),
the weight-average molecular weight (Mw) is from 2,000 to 1,000,000
(inclusive), as measured by a gel permeation chromatography (GPC), and
the glass transition temperature (Tg) is from 40° C. to 80°
C. (inclusive).

[0097]The toner may contain a wax, the amount of which is advantageously
from 0.5 to 20 parts by mass (inclusive) relative to 100 parts by mass of
the binding resin. The peak temperature of the maximum endothermic peak
of the wax is advantageously from 45° C. to 140° C.
(inclusive). The peak temperature within the above-mentioned range is
advantageous, because a toner storage stability and a hot offset property
can be satisfied at the same time.

[0098]Examples of the wax include a hydrocarbon wax such as a
low-molecular weight polyethylene, a low-molecular weight polypropylene,
a paraffin wax, and a Fischer-Tropsch wax; an oxidation product of a
hydrocarbon wax such as oxidized polyethylene wax or their block
copolymer; waxes mainly containing an aliphatic ester, such as a carnauba
wax, a behenyl behenate ester wax, and a montanate ester wax; and partly
or totally deacidified aliphatic esters such as deacidified carnauba wax.

[0099]The use amount of an colorant is advantageously from 0.1 to 30.0
parts by mass (inclusive), more advantageously from 0.5 to 20.0 parts by
mass (inclusive), and most advantageously from 3.0 to 18.0 parts by mass
(inclusive), relative to 100 parts by mass of the binding resin.
Particularly, the amount is from 8.0 to 15.0 parts by mass for a black
toner, from 8.0 to 18.0 parts by mass for a magenta toner, from 6.0 to
12.0 parts by mass for a cyan toner, and from 8.0 to 17.0 parts by mass
for a yellow toner. The use amount within the above-mentioned range is
advantageous in view of dispersibility and chromogenic properties of the
colorant.

[0100]The toner may also contain a charge controlling agent as
appropriate. As the charge controlling agent contained in the toner, a
heretofore known agent may be used, though a colorless metal compound of
an aromatic carboxylic acid having a fast charging rate by friction with
stably keeping the charged electric amount by friction at a certain level
is particularly advantageous.

[0101]Examples of the negative charge controlling agent include a
salicylic acid metal compound, a naphthoic acid metal compound, a
dicarboxylic acid metal compound, a polymer-type compound having a
sulfonic acid or a carboxylic acid in its side chain, a polymer-type
compound having a sulfonic acid salt or a sulfonic acid ester in its side
chain, a polymer-type compound having a carboxylic acid salt or a
carboxylic acid ester in its side chain, a boron compound, an urea
compound, a silicon compound, and a calixarene. The charge controlling
agent may be added internally or externally to the toner particle. The
amount of the charge controlling agent to be added is advantageously from
0.2 to 10.0 parts by mass (inclusive) relative to 100 parts by mass of
the binding resin.

[0102]It is advantageous to add an external additive in order to improve
its fluidity. Inorganic fine particles such as silica, titanium oxide,
and aluminum oxide are advantageous as the external additive. It is
advantageous that the inorganic fine particles be made hydrophobic by a
hydrophobizing agent such as a silane compound, a silicone oil, or a
mixture thereof. The amount of the external additive to be used is
advantageously from 0.1 to 5.0 parts by mass (inclusive) relative to 100
parts by mass of the toner particles. Mixing of the toner particles with
the external additive may be done by using a heretofore known mixer such
as a Henschel mixer.

[0103]Method for producing the toner particles include a crushing method
in which a binding resin and a colorant are melt kneaded, and then the
kneaded mixture is cooled, crushed, and classified; a suspension
granulation method in which a binding resin and a colorant are dissolved
or dispersed in a solvent, the resulting solution is mixed with an
aqueous medium for suspension granulation, and then the solvent is
removed to obtain toner particles; a suspension polymerization method in
which a monomer composition obtained by homogeneously dissolving or
dispersing a monomer, a colorant, and so on is dispersed in a continuous
phase (for example in an aqueous phase) containing a dispersion
stabilizer, and then a polymerization is carried out to obtain toner
particles; a dispersion polymerization method in which toner particles
are formed directly by using a monomer and an aqueous organic solvent
dissolving the monomer but not dissolving the formed polymer; an emulsion
polymerization method in which toner particles are formed directly by
polymerization in the presence of a water-soluble polar polymerization
initiator; and an emulsifying aggregation method including at least a
step of forming an aggregate of fine particles by aggregating polymer
fine particles and colorant fine particles and a step of aging to fuse
the fine particles in the aggregate of fine particles.

[0104]Then, the procedure for toner production by a crushing method will
be described.

[0105]In the step of mixing of raw materials, materials to constitute a
toner particle including a binding resin, a colorant, wax, and, as
appropriate, other components such as a charge controlling agent, are
weighed as intended and then mixed. Examples of the mixing apparatus
include a double cone mixer, a V-shape mixer, a drum mixer, a super
mixer, a Henschel mixer, a Nauta mixer, and Mechano Hybrid (trade name,
manufactured by Mitsui Mining Co., Ltd.).

[0106]Then, the mixed materials are melt kneaded to disperse the colorant
and so on into the binding resin. In the melt kneading step, a batch
kneader such as a pressure kneader and a Bunbury mixer, and a continuous
kneader can be used. Because of the merit of a continuous production, a
monoaxial or a biaxial extruder has been a mainstream. Examples thereof
include a KTK-type biaxial extruder (manufactured by Kobe Steel, Ltd.), a
TEM-type biaxial extruder (manufactured by Toshiba Machine Co., Ltd.), a
PCM melt kneader (manufactured by Ikegai Corp.), a biaxial extruder
(manufactured by KCK K.K.), Ko Kneader (manufactured by Buss AG), and
Kneadex (manufactured by Mitsui Mining Co., Ltd.).

[0107]Then, the colored resin composition obtained by melt kneading may be
rolled by a biaxial roller and the like, and then cooled by water and the
like in the step of cooling.

[0108]The cooled product of the resin composition is crushed until an
intended particle diameter is obtained in a step of crushing, in which
the product is coarsely crushed by a crushing machine such as a crusher,
a hammer mill, and a feather mill, and then pulverized, for example, by a
Criptron system (manufactured by Kawasaki Heavy Industries, Ltd.), a
super rotor (manufactured by Nisshin Engineering Inc.), a turbo mill
(manufactured by Turbo Kogyo Co., Ltd.), and an air jet type pulverizing
mill.

[0109]Thereafter, the toner particle can be obtained, as appropriate, by
classification with a classifying apparatus or a sieve apparatus, such as
an elbow jet using an inertia classification system (manufactured by
Nittetsu Mining Co., Ltd.), Turboprex using a centrifugal classification
system (manufactured by Hosokawa Micron Corp.), TSP Separator
(manufactured by Hosokawa Micron Corp.), and FACULTY (manufactured by
Hosokawa Micron Corp.).

[0110]In addition, after crushing, the toner particle may be
surface-modified, as appropriate, by such treatment as spheronization
using a hybridization system (manufactured by Nara Machinery Co., Ltd.),
a mechanofusion system (manufactured by Hosokawa Micron Corp), FACULTY
(manufactured by Hosokawa Micron Corp.), and Meteo Rainbow MR Type
(manufactured by Nippon Pneumatic Mfg. Co., Ltd.).

[0111]Surface modification of the toner particle may also be done by using
a surface modifying apparatus such as the one shown in FIG. 1. Toner
particles 1 are charged inside a surface modifying apparatus 4 through a
charging nozzle 3 by using an auto feeder 2. An air inside the surface
modifying apparatus 4 is aspirated by a blower 9 so that the toner
particles 1 charged through the charging nozzle 3 are dispersed inside
the apparatus. The toner particles 1 dispersed inside the apparatus are
heated instantaneously by a heated air introduced from a heated air inlet
5 for surface modification. Although it is desirable to generate a heated
air by a heater, the apparatus is not particularly restricted as far as
the apparatus generates a sufficient heated air to for
surface-modification of the toner particles. The surface-modified toner
particles 7 are instantaneously cooled by a cold air introduced from a
cold air inlet 6. Although it is desirable that liquid nitrogen be used
as the cold air, the means is not particularly restricted as far as the
surface-modified toner particles 7 are cooled instantaneously. The
surface-modified toner particles 7 are aspirated by the blower 9 and
collected in a cyclone 8.

[0112]The magnetic carrier of the present invention can be used as a two
component developer containing a magnetic carrier and a toner. When used
as the developer, the mixing ratio is made so that the toner content is
advantageously from 2 to 35 parts by mass (inclusive), and more
advantageously from 4 to 25 parts by mass (inclusive), relative to 100
parts by mass of the magnetic carrier. Within the above-mentioned range,
a high image density can be obtained and scattering of the toner can be
reduced.

[0113]The two component developer of the present invention can also be
used as a replenishing developer used in a two component developing
method in which the developer replenishes a development unit and at least
an overloaded magnetic carrier in the development unit is drained out
from the development unit. When used as a replenishing developer, in
order to increase in durability of the developer, the mixing ratio is
made such that the toner content is advantageously from 2 to 50 parts by
mass (inclusive) relative to 1 part by mass of the magnetic carrier.

[0114]<Method for Measurements of the 50% Particle Diameter on a Volume
Distribution Basis (D50) of the Magnetic Carrier and the Porous Magnetic
Core, the 50% Particle Diameter on a Volume Distribution Basis (D50) of
the Pulverized Product of a Tentatively Calcined Ferrite, and the 90%
Particle Diameter on a Volume Distribution Basis (D90)>

[0116]The measurements of the 50% particle diameter on a volume
distribution basis (D50) of the pulverized product of a tentatively
calcined ferrite and the 90% particle diameter on a volume distribution
basis (D90) are made with an attached sample circulation unit for a wet
method, "Sample Delivery Control (SDC)" (manufactured by Nikkiso Co.,
Ltd.). A tentatively calcined ferrite (ferrite slurry) is added gradually
into the sample circulation unit to obtain an intended concentration for
the measurement. Flow rate of 70%, ultrasonic output power of 40 W, and
ultrasonic dosing time of 60 seconds are employed.

[0119]The measurement of the 50% particle diameter on a volume
distribution basis (D50) of the magnetic carrier and the porous magnetic
core is made with an attached sample charging unit for a dry method,
"one-shot dry type sample conditioner Turbotrac" (manufactured by Nikkiso
Co., Ltd.). Charge to Turbotrac is made by using a dust collector as a
vacuum source with an air flow rate of about liters/second and a pressure
of about 17 kPa. The control is made automatically on the software. The
50% particle diameter (D50) is obtained as an accumulation value on a
volume distribution basis. Control and analysis are made with the
attached software (version of 10.3.3-202D).

[0125]<A Method for Measurements of the Length of the Magnetic Core
Region and the Length of the Region Other than the Magnetic Core Part in
the Cross Section of a magnetic carrier particle, and a method for
measurement of the Area Ratio of the Magnetic Core Region>

[0126]In the process to make a cross section of a magnetic carrier
particle, FB-2100 (manufactured by Hitachi High-Technologies Corp.),
which is a focused ion beam process observation apparatus (FIB), is used.
Carbon paste is applied on a FIB sample stage (metal mesh), and on it a
small amount of magnetic carrier particles are independently adhered one
by one, and then platinum is vapor deposited as a conductive layer to
prepare a sample. The sample is set in the FIB apparatus and roughly
processed by a Ga ion source with an acceleration voltage of 40 kV (beam
current of 39 nA), and then finish-processed (beam current of 7 nA) to
make a cross section of the sample.

[0127]Here, the sample magnetic carrier particles each having the maximum
diameter (Dmax) in the relationship of
D50×0.9≦Dmax≦D50×1.1 are chosen for the
measurement. In addition, Dmax is the maximum diameter when the carrier
particle is observed in a parallel direction from the adhered face. Here,
the distance of the position of the plate containing the maximum length
in the parallel direction with the adhered face of each sample from the
adhered face is taken as "h" (for example, in the case of a perfect
sphere with the radius of "r", h=r). The cross section is made in a
parallel direction with the adhered face in the range from 0.9×h to
1.1×h (inclusive) as the distance from the adhered face.

[0128]The sample processed to have the cross section can be used for
observation with a scanning electron microscope (SEM) as it is. In the
SEM observation, it is known that the more a heavy element, the larger
the amount of the reflected electron emitted from the sample is. For
example, in the case of the sample containing an organic compound and a
metal such as iron distributed in a planar state, the reflected electrons
from iron are detected more so that the part corresponding to iron is
seen bright on the image (high brightness, namely white). On the other
hand, the reflected electrons from the organic compound made of a light
element compound are small so that the image is seen dark (low
brightness, black). In the cross section observation of the magnetic
carrier particle of the present invention, a metal oxide part derived
from the magnetic core region is seen bright (high brightness, white),
and the region other than the magnetic core part is seen dark (low
brightness, black) so that a picture with a large contrast difference
with each other can be obtained.

[0129]Specifically, the observation is made by using a scanning electron
microscope (SEM) S-4800 (manufactured by Hitachi High-Technologies Corp.)
in the following conditions. Here, the observation is made after the
flushing operation.

[0132]Here, the capture of the reflected electron image is made, in
addition to the above-mentioned conditions, by setting the brightness in
the control software of the scanning electron microscope S-4800 at
"Contrast 5, Brightness -5" and the observation mode of magnetic form at
OFF to obtain a gray scale image with 256 gradations.

[0133]The length of the magnetic core region and the length of the region
other than the magnetic core part (resin part and/or void part) in the
cross section of the magnetic carrier particle are calculated by using an
image analysis software Image-Pro Plus 5.1J (manufactured by Media
Cybernetics, Inc.) on the SEM gray-scale reflected electron image of the
cross section of the magnetic carrier particle by the following
procedures.

[0134]Here, one example of the SEM reflected electron image of the
processed cross section of the magnetic carrier particle of the present
invention is shown in FIG. 2. In FIG. 2, a processed cross section region
10 of the magnetic carrier particle, a magnetic core part 11, a resin
part 12, a void part 13, and a magnetic carrier surface 14 are shown.

[0135]Only the processed cross section region 10 of the magnetic carrier
particle is designated on the image in advance. Here, the boundary
between the processed cross section region of the magnetic carrier
particle and the background can be easily distinguished from an observed
reflected electron image. The gray scale image with 256 gradations is
made in the cross section region of the designated particles. The region
is divided into three regions on the picture, namely, a region of the
void part from the 0th to the 10th gradations from the lowest gradation
value, a region of the resin part from the 11th to the 129th gradations,
and a magnetic core region from the 130th to the 254th gradations. The
255th gradation is assigned to a background part other than the processed
cross section region. The processed cross section region of the magnetic
carrier particle is formed of the magnetic core part 11, the resin part
12, and the void part 13, as shown in FIG. 3. Here, the region other than
the magnetic core part is formed of the resin part 12 and the void part
13 in the present invention.

[0136]FIG. 4 shows a schematic drawing of a measurement example
illustrating the magnetic core region and the region other than the
magnetic core part in a cross section of the magnetic carrier particle of
the present invention.

[0137]1. The maximum diameter of the magnetic carrier particle in the
processed cross section region is shown by Rx.

[0138]2. The mid point of Rx is taken as the reference point of the cross
section of the magnetic carrier particle. The diameter perpendicularly
crossing with Rx at the mid point is shown by Ry.

[0139]3. The measurements are made on the magnetic carrier particles
satisfying Rx/Ry≦1.2.

[0140]4. On the 18 lines drawn at intervals of 10° which pass
through the reference point of a cross section of the magnetic carrier
particle, the length and the number are measured on each of the magnetic
core region and the region other than the magnetic core part having a
length of 0.1 μm or longer. From these measurement values, the number
(% by number) of "the magnetic core region having a length of 6.0 μm
or longer relative to the total number of the magnetic core region having
a length of 0.1 μm or longer" and the number (% by number) of "the
region other than the magnetic core part having a length of 4.0 μm or
longer relative to the total number of the region other than the magnetic
core part having a length of 0.1 μm or longer" are taken.

[0141]5. The measurement is repeated for 25 magnetic carriers for the
particles satisfying Rx/Ry≦1.2, and the average of them is
calculated. The ratio of the particles satisfying Rx/Ry≦1.2 is
calculated by using the number of particles necessary to reach 25 in the
cutting process as the denominator.

[0142]In FIG. 5, an example is shown for the distribution of the length
and the numbers (% by number) obtained by measuring, in the method as
mentioned above, the magnetic core region having a length of 0.1 μm or
longer and the region other than the magnetic core part having a length
of 0.1 μm or longer in the cross section of the magnetic carrier
particle of the present invention.

[0143]In the measurement of the area ratio of the magnetic core part in
the cross section of the magnetic carrier particle, the processed cross
section region of the magnetic carrier particle is assigned in advance as
the cross section area of the magnetic carrier particle. The value
obtained by dividing the area occupied by the magnetic core part 1 by the
cross section area of the magnetic carrier particle is taken as the "area
ratio (% by area) of the magnetic core part". In the present invention,
the same measurements are done for the 25 magnetic carrier particles as
mentioned above to obtain the average value for use.

[0144]<Measurements of the Weight-Average Particle Diameter (D4) of the
Toner, the Percent by Number of the Particles with a Diameter of 4.0
μm or Less, and the Percent by Volume of the Particles Having a
Diameter of 12.7 μm or More>

[0145]The weight-average particle diameter (D4) of the toner is obtained
by calculating the data obtained as following. Namely, the measurements
are made with a precision particle size distribution measurement
apparatus by a micro pore electric resistance method equipped with a 100
μm aperture tube, "Coulter Counter Multisizer 3" (trade name,
manufactured by Beckman Coulter, Inc.), with the effective measurement
channels of 25,000, wherein setting of the measurement conditions and the
data analysis from the measurements are done with the dedicated software
attached thereto, "Beckman Coulter Multisizer 3 Version 3.51"
(manufactured by Beckman Coulter, Inc.).

[0146]The aqueous electrolyte solution obtained by dissolving the special
grade sodium chloride into an ion-exchanged water (concentration of about
1% by mass), for example, "ISOTON II" (manufactured by Beckman Coulter,
Inc.), can be used for the measurement.

[0147]Prior to the measurement and the analysis, the dedicated software is
set as following. In the screen "change of standard operation mode
(SOM)", the number 50,000 is set as the total count numbers of the
control mode of the particles with one time measurement. The value
obtained by "the standard particle of 10.0 μm" (manufactured by
Beckman Coulter, Inc.) is set as the Kd value. By pressing the
measurement button of the threshold/noise level, the threshold and the
noise level are automatically set. The settings are made at 1,600 μA
for the current, 2 for the gain, and ISOTON II for the electrolyte
solution. The check is made on the flush of the aperture tube after the
measurement. In the screen "setting of change from pulse to particle
diameter" of the above-mentioned dedicated software, the logarithmic
particle diameter is set for the bin distance, the particle diameter bin
is set for the 256 particle diameter bin, and the particle diameter range
is set from 2 μm to 60 μm.

[0148]A specific measurement method is as following.

[0149](1) About 200 mL of the above-mentioned aqueous electrolyte solution
is taken into a 250-mL round bottom glass beaker dedicated to Multisizer
3, and then the beaker is set on a sample stand. A stirring rod is
rotated counterclockwise at the rate of 24 rotations/second. With the
function of the "aperture flush" in the analysis software, blots and air
bubbles in the aperture tube are removed.

[0150](2) Into a 100-mL flat bottom glass beaker is taken about 30 mL of
the above-mentioned aqueous electrolyte solution, and then about 0.3 mL
of a solution obtained by diluting "Contaminon N" (manufactured by Wako
Pure Chemical Industries, Ltd.; a 10% by mass aqueous neutral detergent
solution with pH 7 formed of a nonionic surfactant, an anionic
surfactant, and an organic builder for washing of a precision measurement
apparatus) with an ion-exchanged water by three folds by mass.

[0151](3) A prescribed amount of an ion-exchanged water is charged into a
water bath of an ultrasonic disperser "Ultrasonic Dispersion System
Tetoral 150" (manufactured by Nikkaki-Bios Co., Ltd.) of an electric
output power of 120 W, which has, inside the apparatus, two oscillators
having the oscillation frequency of 50 kHz in the sate of phase
difference of 180 degrees. And then, about 2 mL of the above-mentioned
Contaminon N is added into the water bath.

[0152](4) The beaker mentioned in (2) is set in the beaker-holding hole in
the above-mentioned ultrasonic disperser, and then the ultrasonic
disperser is started. The height position of the beaker is adjusted so
that the co-vibration of the surface of the aqueous electrolyte solution
in the beaker becomes the maximum.

[0153](5) About 10 mg of a toner is added little by little into the
aqueous electrolyte solution in the beaker mentioned in (4) under dosing
of the ultrasonic wave to the aqueous electrolyte solution for
dispersion. The dispersion treatment by the ultrasonic wave is continued
for further 60 seconds. Here, the temperature of the water in the water
bath during the ultrasonic dispersion is controlled appropriately in the
range from 10° C. to 40° C. (inclusive).

[0154](6) The aqueous electrolyte solution obtained in (5) containing the
dispersed toner is added dropwise with a pipette into the round bottom
beaker in (1) set on the sample stand to obtain a solution with the
measurement concentration of about 5%. And then the measurements are
continued until the number of measured particles reaches 50,000.

[0155](7) The measured data are analyzed by the above-mentioned dedicated
software attached to the apparatus to calculate the weight-average
particle diameter (D4). The "average diameter" shown in the screen
analysis/volume statistics number (arithmetic mean) when the graph/volume
% is set in the above-mentioned dedicated software is the weight-average
particle diameter (D4).

[0156]The percentage by number of the particles having a diameter of 4
μm or less in the toner is calculated by analyzing the data after
measurements by the above-mentioned Multisizer 3. Firstly, the graph/% by
number is set by the above-mentioned dedicated software, and the chart of
the measurement results is set at the % by number display. Then, the mark
"<" in the particle diameter setting part in the screen
"format/particle diameter/particle diameter statistics" is checked, and
then the number "4" is entered in the particle diameter entry part
thereunder. The number appearing in the display part "<4 μm", when
the screen "analysis/number statistics (arithmetic mean)" is displayed,
is the percentage by number of the particles having a diameter of 4.0
μm or less in the toner.

[0157]The percentage by volume of the particles having a diameter of 12.7
μm or more on a volume basis in the toner is calculated by analyzing
the data after the above-mentioned Multisizer 3 measurements. Firstly,
the graph/% by volume is set by the above-mentioned dedicated software,
and the chart of the measurement results is set at the % by volume
display. Then, the mark ">" in the particle diameter setting part in
the screen "format/particle diameter/particle diameter statistics" is
checked, and then the number "12.7" is entered in the particle diameter
entry part thereunder. The number appearing in the display part ">12.7
μm", when the screen "analysis/volume statistics (arithmetic mean)" is
displayed, is the percentage by volume of the particles having a diameter
of 12.7 μm or more in the toner.

[0158]<Average Circularity of the Toner>

[0159]The average circularity of the toner is measured by a flow-type
particle image analysis apparatus "FPIA-3000 Type" (manufactured by
Sysmex Corp.) under the conditions of measurement and analysis used at
the time of calibration. The circle equivalent diameter and the
circularity are obtained by using the projected area "S" and the
periphery length "L". The circle equivalent diameter is meant by the
diameter of a circle having the same area as the projected area in the
particle image. The circularity is defined as the value which is obtained
by dividing the periphery length of the circle obtained from the circle
equivalent diameter by the periphery length of the projected particle
image and can be calculated by the following equation.

Circularity C=2×(π×S)1/2/L

[0160]The circularity is 1.000 when the particle image is a true circle,
and is smaller when the degree of asperity in the periphery of the
particle image is larger. After calculating the circularity of each
particle, the range of the circularity from 0.2 to 1.0 (inclusive) is
divided into 800 channels, and the median value of each channel is taken
as the representative value, from which the average value is calculated
to obtain the average circularity.

[0161]In the specific measurement method, after a surfactant as a
dispersing agent, advantageously 0.02 g of sodium dodecylbenzene
sulfonate, is added to 20 mL of ion-exchanged water, 0.02 g of a
measurement sample is added. Then, the resulting mixture is treated for
dispersion for 2 minutes by using a table-top ultrasonic cleaning
disperser with an oscillation frequency of 50 kHz and an electric output
power of 150 W (for example, "VS-150" manufactured by Velvo-Clear Co.) to
obtain a disperse solution for the measurement. During the operation, the
temperature of the disperse solution is cooled in the range from
10° C. to 40° C. (inclusive) appropriately.

[0162]At the measurement, the above-mentioned flow-type particle image
analysis apparatus mounted with a regular objective lens (10 times
magnification) is used with a sheath solution, the particle sheath
"PSE-900A" (manufactured by Sysmex Corp.). The disperse solution prepared
according to the above-mentioned procedure is introduced into the
flow-type particle image analysis apparatus, and 3,000 toner particles
are measured with the HPF measurement mode and the total count mode. The
average circularity of the toner is obtained by setting the binarization
threshold at the time of particle analysis at 85% while the circle
equivalent diameter of the particle diameter for analysis is limited from
2.00 to 200.00 μm (inclusive).

[0163]At the measurement, adjustment of an automatic focus is made by
using the standard latex particle (for example, 5200A, manufactured by
Duke Scientific Corp., diluted with ion-exchanged water) prior to the
measurement. Thereafter, it is advantageous to carry out the focus
adjustment every two hours after start of the measurement.

[0164]In Examples of the present application, the flow-type particle image
analysis apparatus with the proof certificate issued by a Sysmex Corp.
was used. The measurements were made under the measurement and analysis
conditions described in the proof certificate except that the circle
equivalent diameter of the particle diameter for analysis was limited to
the range from 2.00 to 200.00 μm (inclusive).

[0165]<A Method for Measurements of the Peak Molecular Weight (Mp), the
Number-Average Molecular Weight (Mn), and the Weight-Average Molecular
Weight (Mw) of the THF-Soluble Fraction of the Resin or the Toner>

[0166]The peak molecular weight (Mp), the number-average molecular weight
(Mn), and the weight-average molecular weight (Mw) are measured as
following by using a gel permeation chromatography (GPC). Firstly, a
sample is dissolved into tetrahydrofurane (THF) at room temperature in 24
hours. The sample to be used is a resin or a toner. Thus obtained
solution is filtered through "Myshori Disk" (manufactured by Tosoh
Corp.), a solvent-resistant membrane filter with a pore diameter of 0.2
μm, to obtain a sample solution. Here, the sample solution is prepared
so that the concentration of the THF-soluble fraction is about 0.8% by
mass. By using this sample solution, measurements are made under the
following conditions.

<Peak Temperature of the Maximum Endothermic Peak of the Wax, and Glass
Transition Temperature Tg of the Binding Resin or the Toner>

[0168]The peak temperature of the maximum endothermic peak of the wax is
measured by using a differential scanning calorimeter "Q 1000"
(manufactured by TA Instruments, Inc.) in accordance with ASTM D3418-82.
Temperature correction in the apparatus detector part is made with
melting points of indium and zinc. Correction of the heat quantity is
made with a heat of melting of indium.

[0169]Specifically, about 10 mg of wax is accurately weighed, put on a pan
made of aluminum, and then measured at the heating rate of 10°
C./minute in the measurement temperature range from 30 to 200° C.
(inclusive) with a reference of an empty pan made of aluminum. Here, in
the measurement, the temperature is raised to 200° C. once, cooled
to 30° C., and then raised again. The maximum endothermic peak in
the DSC curve of the second heating process in the temperature range from
30 to 200° C. (inclusive) is taken as the peak temperature of the
maximum endothermic peak of the wax in the present invention. The glass
transition temperature (Tg) of the binding resin or the toner is measured
by using about 10 mg of an accurately weighed binding resin or toner in a
similar manner to the measurement of the peak temperature of the maximum
endothermic peak of the wax. Then, a change in specific heat is obtained
in the temperature range from 40° C. to 100° C.
(inclusive). The intersection point of the line drawn between the mid
points of the base lines before and after the change in specific heat and
the differential thermal curve is taken as the glass transition
temperature (Tg) of the binding resin or the toner.

EXAMPLES

Production Example 1 of the Porous Magnetic Core

[0170]Fe2O3: 56.1% by massMnCO3: 35.8% by massMg(OH)2:
6.9% by massSrCO3: 1.2% by mass

[0171]Each of the above materials was weighed to form a ferrite raw
material having the above composition.

[0172]Then, they were crushed and mixed by a dry-type ball mill using
zirconia balls with 10 mm diameter (φ) for hours (Step 1: the
weighing and mixing step). After crushing and mixing, the resulting
mixture was calcined in an atmospheric air by a burner-type calcination
furnace at 950° C. for 2 hours to obtain a tentatively calcined
ferrite (Step 2: the tentative calcination step). The composition of the
ferrite is as following:

(MnO)a(MgO)b(SrO)c(Fe2O3)d

wherein, a=0.395, b=0.150, c=0.010, and d=0.445.

[0173]The tentatively calcined ferrite was crushed to a size of about 0.3
mm by a crusher, and then crushed in a wet-type ball mill by using
stainless steel balls with a diameter (φ) of 10 mm with adding 30
parts by mass of water relative to 100 parts by mass of the tentatively
calcined ferrite for one hour. Thus obtained slurry was crushed in a
wet-type bead mill by using zirconia beads with a diameter (φ) of 1.0
mm for one hour to obtain a ferrite slurry (pulverized product of
tentatively calcined ferrite) (Step 3: the crushing step). Thus obtained
pulverized product of tentatively calcined ferrite showed 2.0 μm as
the 50% particle diameter (D50) on a volume distribution basis, 6.4 μm
as the 90% particle diameter (D90) on a volume distribution basis, and
3.2 as D90/D50.

[0174]To the ferrite slurry, 2.0 parts by mass of polyvinyl alcohol
relative to 100 parts by mass of the tentatively calcined ferrite was
added as a binder, and then the resulting mixture was granulated to
spherical particles by a spray dryer (manufactured by Okawara Corp.)
(Step 4: the granulation step). In an electric furnace, the temperature
was raised under the nitrogen atmosphere (1.0% by volume of oxygen
concentration) from a room temperature to 1,100° C. during 3 hours
and then the calcination was done at 1,100° C. for 4 hours.
Thereafter, the temperature was lowered to 80° C. during 8 hours,
the nitrogen atmosphere was returned to an atmospheric air, and then the
particles were taken out at the temperature of 40° C. or lower
(Step 5: the calcination step). After the aggregated particles were
parted, they were sieved with a sieve having an opening of 250 μm for
removal of coarse particles to obtain the porous magnetic core 1 with the
50% particle diameter (D50) of 29.7 μm on a volume distribution basis
(Step 6: the classification step). The obtained physical properties are
shown in Table 1.

Production Example 2 of the Porous Magnetic Core

[0175]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 3, the degree of
crushing particles in the crusher was changed from about 0.3 mm to about
0.5 mm, the balls in the wet-type ball mill were changed from stainless
steel with a 10 mm diameter (φ) to zirconia with a 10 mm diameter
(φ), and the crushing time was changed from one hour to two hours.
The crushing time in the wet-type bead mill was changed from one hour to
two hours. In Step 5, the calcination temperature was changed from
1,100° C. to 1,050° C. and the time for raising the
temperature from a room temperature to the calcination temperature was
changed from 3 hours to 2 hours. The other conditions were made as same
as those in the production example 1 of the porous magnetic core to
obtain the porous magnetic core 2. The obtained physical properties are
shown in Table 1.

Production Example 3 of the Porous Magnetic Core

[0176]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 3, the degree of
crushing particles in the crusher was changed from about 0.3 mm to about
0.5 mm, the balls in the wet-type ball mill were changed from stainless
steel with a 10 mm diameter (φ) to zirconia with a 10 mm diameter
(φ), and the crushing time was changed from one hour to two hours.
The crushing time in the wet-type bead mill was changed from one hour to
three hours. In Step 4, 2.0 parts by mass of sodium carbonate was added
as a pore controlling agent along with 2.0 parts by mass of polyvinyl
alcohol as a binder to the ferrite slurry. In Step 5, the calcination
temperature was changed from 1,100° C. to 1,050° C. The
other conditions were made as same as those in the production example 1
of the porous magnetic core to obtain the porous magnetic core 3. The
obtained physical properties are shown in Table 1.

Production Example 4 of the Porous Magnetic Core

[0177]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 3, the degree of
crushing particles in the crusher was changed from about 0.3 mm to about
0.5 mm, the balls in the wet-type ball mill were changed from stainless
steel with a 10 mm diameter (φ) to zirconia with a 10 mm diameter
(φ), and the crushing time was changed from one hour to three hours.
The beads in the wet-type bead mill were changed from zirconia with a 1.0
mm diameter (φ) to alumina with a 1.0 mm diameter (φ) and the
crushing time was changed from one hour to two hours. In Step 4, 0.5
parts by mass of sodium carbonate was added as a pore controlling agent
along with 2.0 parts by mass of polyvinyl alcohol as a binder to the
ferrite slurry. In Step 5, the calcination temperature was changed from
1,100° C. to 1,050° C. and the calcination time was changed
from 4 hours to 2 hours. The other conditions were made as same as those
in the production example 1 of the porous magnetic core to obtain the
porous magnetic core 4. The obtained physical properties are shown in
Table 1.

Production Example 5 of the Porous Magnetic Core

[0178]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 1, the ratio of the
ferrite raw materials was changed to the following:

Fe2O3: 61.3% by massMnCO3: 31.0% by massMg(OH)2: 7.7%
by mass

[0179]In Step 3, the crushing time was changed from one hour to two hours.
The beads in the wet-type bead mill were changed from zirconia with a 1.0
mm diameter (φ) to stainless steel with a 1.0 mm diameter (φ) and
the crushing time was changed from one hour to two hours. In Step 4, the
amount of polyvinyl alcohol added as a binder was changed from 2.0 parts
by mass to 1.0 parts by mass. In Step 5, the calcination temperature was
changed from 1,100° C. to 1,200° C. and the calcination
time was changed from 4 hours to 6 hours. The other conditions were made
as same as those in the production example 1 of the porous magnetic core
to obtain the porous magnetic core 5. The obtained physical properties
are shown in Table 1.

Production Example 6 of the Porous Magnetic Core

[0180]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 1, the ratio of the
ferrite raw materials was changed to the following:

Fe2O3: 60.7% by massMnCO3: 32.0% by massMg(OH)2: 6.4%
by massSrCO3: 0.9% by mass

[0181]In Step 3, the beads in the wet-type bead mill were changed from
zirconia with a 1.0 mm diameter (φ) to stainless steel with a 1.0 mm
diameter (φ) and the crushing time was changed from one hour to four
hours. The time for raising the temperature from a room temperature to
the calcination temperature was changed from 3 hours to 5 hours. The
other conditions were made as same as those in the production example 1
of the porous magnetic core to obtain the porous magnetic core 6. The
obtained physical properties are shown in Table 1.

Production Example 7 of the Porous Magnetic Core

[0182]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 1, the ratio of the
ferrite raw materials was changed to the following:

Fe2O3: 60.8% by massMnCO3: 24.0% by massMg(OH)2: 14.2%
by massSrCO3: 1.0% by mass

[0183]In step 2, the temperature for the tentative calcination was changed
from 950° C. to 900° C.

[0184]In Step 3, the degree of crushing particles in the crusher was
changed from about 0.3 mm to about 0.5 mm, the balls in the wet-type ball
mill were changed from stainless steel with a 10 mm diameter (φ) to
alumina with a 10 mm diameter (φ), and the crushing time was changed
from one hour to four hours. Crushing by the wet-type bead mill was not
carried out. In Step 4, 4.0 parts by mass of sodium carbonate was added
as a pore controlling agent along with 4.0 parts by mass of polyvinyl
alcohol as a binder to the ferrite slurry. In Step 5, the calcination
temperature was changed from 1,100° C. to 1,250° C. and the
calcination time was changed from 4 hours to 5 hours. The other
conditions were made as same as those in the production example 1 of the
porous magnetic core to obtain the porous magnetic core 7. The obtained
physical properties are shown in Table 1.

Production Example 8 of the Porous Magnetic Core

[0185]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 1, the ratio of the
ferrite raw materials was changed to the following:

Fe2O3: 95.4% by massLi2CO3: 4.6% by mass

[0186]In Step 3, the crushing time in the wet-type bead mill was changed
from one hour to 20 hours. In Step 5, the calcination temperature was
changed from 1,100° C. to 1,150° C. The other conditions
were made as same as those in the production example 1 of the porous
magnetic core to obtain the porous magnetic core 8. The obtained physical
properties are shown in Table 1.

Production Example 9 of the Magnetic Core

[0187]Fe2O3: 73.3% by massCuO: 12.2% by massZnO: 14.5% by mass

[0188]Each of the above materials was weighed to form a ferrite raw
material having the above-mentioned composition. Then, they were crushed
and mixed by a dry-type ball mill using zirconia balls with 10 mm
diameter (φ) (Step 1: the weighing and mixing step) for 2 hours.
After crushing and mixing, the resulting mixture was calcined in an
atmospheric air at 950° C. for 2 hours to obtain a tentatively
calcined ferrite (Step 2: the tentative calcination step). After crushed
to a size of about 0.5 mm by a crusher, the crushing was done in a
wet-type ball mill by using stainless steel balls with a diameter (φ)
of 10 mm with adding 30 parts by mass of water relative to 100 parts by
mass of the tentatively calcined ferrite for 6 hours (Step 3: the
crashing step). To the ferrite slurry, 2.0 parts by mass of polyvinyl
alcohol relative to 100 parts by mass of the tentatively calcined ferrite
was added as a binder, and then the resulting mixture was granulated to
spherical particles by a spray dryer (manufactured by Okawara Corp.)
(Step 4: the granulation step). The temperature was raised in an
atmospheric air from a room temperature to the calcination temperature
during 3 hours, and then the calcination was done at 1,300° C. for
4 hours. Thereafter, the temperature was lowered to 40° C. during
6 hours, and then the particles were taken out (Step 5: the calcination
step). After the aggregated particles were parted, they were sieved with
a sieve having an opening of 250 μm for removal of coarse particles to
obtain the magnetic core 9 (Step 6: the classification step). The
obtained physical properties are shown in Table 1.

Production Example 10 of the Porous Magnetic Core

[0189]In the production example 1 of the porous magnetic core, the
following conditions were changed. Namely, in Step 1, the ratio of the
ferrite raw materials was changed to the following:

Fe2O3: 61.8% by massMnCO3: 31.1% by massMg(OH)2: 6.5%
by massSrCO3: 0.6% by mass

[0190]In step 3, the beads in the wet-type bead mill was changed from the
zirconia with a diameter (φ) of 1.0 mm to the stainless steel with a
diameter (φ) of 1/8 inch, and the crushing was done for one hour.
Then, the crushing was further done by using stainless steel beads with a
diameter (φ) of 1/16 inch for four hours. In Step 4, the amount of
polyvinyl alcohol used as a binder was changed from 2.0 parts by mass to
1.0 part by mass. In Step 5, the time for raising the temperature from a
room temperature to the calcination temperature was changed from 3 hours
to 5 hours, and the atmosphere was changed to nitrogen with the oxygen
concentration of 0% by volume. The other conditions were made as same as
those in the production example 1 of the porous magnetic core to obtain
the porous magnetic core 10. The obtained physical properties are shown
in Table 1.

[0202]Nitrogen was introduced under reduced pressure to a mixing stirrer
(versatile stirrer NDMV-type, manufactured by Dalton Co., Ltd.)
containing 100.0 parts by mass of the porous magnetic core 1 with keeping
a temperature at 30° C., and then 13.0 parts by mass (as a resin
component, relative to the porous magnetic core 1) of the resin solution
1 was added dropwise into it under reduced pressure. The agitation of the
resulting mixture was continued as it was for 2 hours after completion of
the dropwise addition. Thereafter, the temperature was raised to
70° C., and then the solvent was removed under reduced pressure to
fill inside the core particles of the porous magnetic core 1 with the
silicone resin composition. After cooling, thus obtained magnetic carrier
particles were transferred to a mixer having a spiral blade inside a
rotatable mixing vessel (drum mixer UD-AT type, manufactured by Sugiyama
Heavy Industrial Co., Ltd.), heat-treated at 200° C. under a
nitrogen atmosphere for 2 hours, and then classified by a sieve with an
opening of 70 μm to obtain the magnetic core.

[0203]Step 2 (the Resin Coating Step):

[0204]This magnetic core (100.0 parts by mass) was taken into a fluidized
bed coating apparatus (Spiraflow SFC type, manufactured by Freund Corp.),
and then nitrogen with a charging temperature of 80° C. was
charged at the flow rate of 0.8 m3/minute. Rotation speed of a
rotating rotor was made 1,000 rotations per minute, and after the product
temperature reached 50° C., spraying of the resin solution 2 was
started. The spraying rate was made at 3.5 g/minute. The coating was
continued until the amount of the coated resin reached 0.8 parts by mass
relative to 100.0 parts by mass of the above-mentioned magnetic core.

[0205]Thereafter, the magnetic core coated with the silicone resin was
transferred to a mixer having a spiral blade inside a rotatable mixing
vessel (drum mixer UD-AT type, manufactured by Sugiyama Heavy Industrial
Co., Ltd.), and then heat-treated at 200° C. under a nitrogen
atmosphere for 2 hours with rotating the mixing vessel at the rate of 10
rotations per minute for agitation. By agitation, the resin thickness
state on surface of the magnetic carrier particles was controlled. Thus
obtained magnetic carrier particles were passed through a sieve with an
opening of 70 μm to obtain the magnetic carrier 1. The kind and the
amount of the resin in the magnetic carrier 1 in the resin filling step
and the resin coating step are shown in Table 2.

Production Examples of the Magnetic Carriers 2 to 11

[0206]The kind and the amount of the filling resin in the resin filling
step, and the kind and the amount of the resin in the resin coating step
were changed as shown in Table 2 to obtain the magnetic carriers 2 to 11.

Production Example of the Magnetic Carrier 12

[0207]Step 1 (the Resin Filling Step):

[0208]Into a monoaxial indirect heating dryer (Torusdisk TD type,
manufactured by Hosokawa Micron Corp.) containing 100.0 parts by mass of
the porous magnetic core was added dropwise 20.0 parts by mass (as a
resin component, relative to the porous magnetic core 10) of the resin
solution 4 while charging nitrogen and keeping a temperature at
75° C. The agitation of the resulting mixture was continued as it
was for 2 hours after completion of the dropwise addition. Thereafter,
the temperature was raised to 200° C., and then the solvent was
removed under reduced pressure. After heating at 200° C. for hours
and then cooling, the magnetic carrier 12 was obtained by classification
with a sieve having an opening of 70 μm. Step 2 (the resin coating
step) was not carried out.

Production Example of the Magnetic Carrier 13

[0209]In the production example of magnetic carrier 12, the filling amount
in Step 1 was changed from 20.0 parts by mass to 13.0 parts by mass.
Further, in Step 2, 100.0 parts by mass of the magnetic carrier 12 was
taken into a fluidized bed coating apparatus (Spiraflow SFC type,
manufactured by Freund Corp.), and then nitrogen with a charging
temperature of 70° C. was charged at the flow rate of 0.8
m3/minute. Rotation speed of a rotating rotor was made 1,000
rotations per minute, and after the product temperature reached
50° C., spraying of the resin solution 5 was started. The spraying
rate was made at 3.5 g/minute. The coating was continued until the amount
of the coated resin reached 2.0 parts by mass relative to 100.0 parts by
mass of the magnetic carrier 12. Then, the dryer was changed to a vacuum
dryer and then the heat-treatment after coating was done under reduced
pressure (about 0.01 MPa) with flowing nitrogen at the rate of 0.01
m3/minute at 220° C. for 2 hours to obtain the magnetic
carrier 13.

[0210]Physical properties of the obtained magnetic carriers 1 to 13 are
shown in Table 3.

[0212]Thereafter, the resulting mixture was reacted by heating at
210° C. under nitrogen stream for 9 hours while removing produced
water. Then, 61 parts by mass of trimellitic anhydride was added and the
heating was continued at 170° C. for 3 hours to obtain the resin
1. The resin 1 had the weight-average molecular weight (Mw) of 68,000,
the number-average molecular weight (Mn) of 5,700, and the peak molecular
weight (Mp) of 10,500, as obtained by GPC measurement, and the glass
transition temperature (Tg) of 61° C.

[0214]Then, 100 parts by mass of the pulverized product 1 was mixed with
1.0 part by mass of AEROSIL R972 (manufactured by Nippon Aerosil Co.,
Ltd.) in a Henschel mixer (FM-75 Type, manufactured by Mitsui Miike
Kakoki K. K.). Thus obtained mixture was surface-modified by a surface
modifying apparatus shown in FIG. 1. The surface modification was carried
out under the conditions with the charging rate of the raw materials at
2.0 kg/hour and the blowing temperature of the heated air at 210°
C. Then, fine particles and coarse particles were removed simultaneously
by an air wind classifier using Coanda effect (Elbojet Labo EJ-L3,
manufactured by Nittetsu Mining Co., Ltd.) to obtain the toner particles
1. Thus obtained toner particles 1 (100.0 parts by mass) were mixed with
external additives, 1.0 part by mass of STT-30A (manufactured by Titan
Kogyo, Ltd.) and 1.0 part by mass of AEROSIL R972 (Nippon Aerosil Co.,
Ltd.) to obtain the toner 1. Properties of the toner 1 were as following;
6.2 μm as the weight-average particle diameter (D4), 21.3% by number
of the particles having a diameter of 4.0 μm or less on a number
basis, 1.0% by volume of the particles having a diameter of 12.7 μm or
more on a volume basis, and 0.969 as the average circularity.

Production Example of the Toner 2

[0215]In the production example of toner 1, the obtained pulverized
product 1 was treated by a particle design apparatus (product name of
FACULTY, manufactured by Hosokawa Micron Corp.), which was modified in
shape and number of the hammer, for simultaneous classification and
spheronization to obtain the toner particles 2. Other than the
above-mentioned, the same operation as the production example of the
toner 1 was followed to obtain the toner 2. Properties of the toner 2
were as following; 5.5 μm as the weight-average particle diameter
(D4), 27.6% by number of the particles having a diameter of 4.0 μm or
less on a number basis, 0.4% by volume of the particles having a diameter
of 12.7 μm or more on a volume basis, and 0.950 as the average
circularity.

[0217]Separately, 900 parts by mass of 0.1 M Na3PO4 aqueous
solution was added to 710 parts by mass of ion-exchanged water. After the
resulting mixture was heated to 60° C., 67.7 parts by mass of 1.0
M CaCl2 aqueous solution was gradually added into the mixture to
obtain an aqueous medium containing a calcium phosphate compound.

[0218]Then, a mixture of 40.0 parts by mass of the master batch disperse
solution 1, 67.0 parts by mass of styrene monomer, 19.0 parts by mass of
n-Butyl acrylate monomer, 12.0 parts by mass of ester wax (endothermic
peak temperature of 66° C.), 0.2 parts by mass of divinyl benzene,
and 5.0 parts by mass of saturated polyester (polycondensation product of
bisphenol A propyleneoxide adduct, terephthalic acid, and trimellitic
anhydride; Mp=11,000) was heated to 55° C., and dissolved and
dispersed homogeneously by a TK-type homomixer (manufactured by Tokushu
Kika Kogyo Co., Ltd.) at 83.3 S-1 (5,000 rpm). Into this mixture was
dissolved 3.5 parts by mass of 2,2'-azobis(2,4-dimethylvaleronitrile) (a
polymerization initiator) to obtain a monomer composition. The monomer
composition was charged into the above-mentioned aqueous medium and the
resulting mixture was agitated in the TK-type homomixer at 233.3 s-1
(14,000 rpm) and 60° C. under a nitrogen atmosphere to granulate
the monomer composition.

[0219]Thereafter, the composition was agitated with a paddle agitator for
5 hours. After the temperature was raised to 80° C. at the heating
rate of 40° C./hour, the reaction was carried out for 5 hours with
agitation. After termination of the polymerization, residual monomers
were removed by evaporation under reduced pressure. After cooled,
hydrochloric acid was added to adjust the pH at 1.4, and then the calcium
phosphate salt was dissolved by agitating the resulting mixture for 6
hours. Thereafter, the mixture was filtered, washed by ion-exchanged
water, and then dried to obtain the toner particles 3.

[0220]Other than the above-mentioned, the same operation as the production
example of the toner 1 was followed to obtain the toner 3 having the
following properties; 4.5 μm as the weight-average particle diameter
(D4), 33.1% by number of the particles having a diameter of 4.0 μm or
less on a number basis, 0.0% by volume of the particles having a diameter
of 12.7 μm or more on a volume basis, and 0.991 as the average
circularity. Molecular weights of the THF-soluble fraction of the toner 3
obtained by GPC were as following; 40,000 as the weight-average molecular
weight (Mw), 11,500 as the number-average molecular weight (Mn), and
28,000 as the peak molecular weight (Mp).

Production Example of the Toner 4

[0221]In the production example of the toner 1, the obtained pulverized
product 1 was classified by an air wind classifier Elbojet (manufactured
by Nittetsu Mining Co., Ltd.) to obtain the toner particles 4. Properties
of the toner particles 4 were as following; 5.1 μm as the
weight-average particle diameter (D4), 34.8% by number of the particles
having a diameter of 4.0 μm or less on a number basis, 0.6% by volume
of the particles having a diameter of 12.7 μm or more on a volume
basis, and 0.939 as the average circularity. Other than the
above-mentioned, the same operation as the production example of the
toner 1 was followed to obtain the toner 4.

Production Example of the Toner 5

[0222]In the production example of the toner 1, the obtained coarsely
crushed product 1 was made to the pulverized product 2 by using a
collision-type air jet pulverizing mill with a high pressure air. Thus
obtained pulverized product 2 was classified by an air wind classifier
Elbojet (manufactured by Nittetsu Mining Co., Ltd.) to obtain the toner
particles 5. Properties of the toner particles 5 were as following; 8.9
μm as the weight-average particle diameter (D4), 11.7% by number of
the particles having a diameter of 4.0 μm or less on a number basis,
5.2% by volume of the particles having a diameter of 12.7 μm or more
on a volume basis, and 0.932 as the average circularity. Other than the
above-mentioned, the same operation as the production example of the
toner 1 was followed to obtain the toner 5.

[0223]The physical properties of the toners 1 to 5 are shown in Table 4.

[0224]The prepared magnetic carrier and toner were combined as shown in
Table 5 to obtain the two component developer. The two component
developer was made by mixing them by a V-shape mixer for 5 minutes in the
ratio of 90.0% by mass of the magnetic carrier and 10.0% by mass of the
toner. Thus obtained two component developer was evaluated by the
following methods, and the results are shown in Table 6.

[0225]To use as the image forming apparatus, a commercially used digital
printer imagePRESS C1 (manufactured by Canon, Inc.) was modified, and
using this, the image was formed for evaluation by charging the
above-mentioned developer into the cyan position of the development unit.
Here, the modification was made so that the mechanism that would
discharge an excessive magnetic carrier in the development unit from the
development unit was removed and an alternate current voltage with 2.0
kHz frequency and 1.3 kV Vpp and a direct current voltage VDC were
applied to the developer carrier. The direct current voltage Vcc was
controlled so that the mounting amount of the toner of the FFh image
(solid image) on a sheet of paper would be 0.6 mg/cm2. Here, the FFh
image is the value showing the 256 gradations by the hexadecimal, wherein
the first gradation of 256 gradations (white part) is taken as 00h and
the 256th gradation of 256 gradations (solid part) is taken as FFh. Under
the above-mentioned conditions, the 50,000 copies durability test with
the image ratio of 5% was carried out by using the original script (A4)
of the FFh image to evaluate the following items.

[0228]A dot image (FFh image) formed with one pixel by one dot was
prepared. The spot diameter of a laser beam was adjusted so that the area
per dot on a sheet of paper would be from 20,000 μm2 to 25,000
μm2 (inclusive). The area of 1,000 dots was measured by using a
digital microscope VHX-500 (wide range zoom lens VH-Z100, manufactured by
Keyence Corp.). The number-average of the dot area (S) and the standard
deviation of the dot area (G) were calculated and the dot reproducibility
index was calculated by the following equation.

Dot reproducibility index (I)=σ/S×100

Wherein,

[0229]A: I is less than 4.0B: I is 4.0 or more and less than 6.0C: I is
6.0 or more and less than 8.0D: I is 8.0 or more

[0230]<Fogging>

[0231]At N/N and H/H, 10 sheets of paper of the 00h image were printed
out, and the average reflectance Dr (%) of the 10th copy was measured by
a reflectometer (Reflectomer Model TC-6DS, manufactured by Tokyo Denshoku
Co., Ltd.). On the other hand, the reflectance Ds (%) of the paper
without image output was measured. The fog (%) was calculated by the
following equation.

Fog (%)=Dr(%)-Ds(%)

Wherein,

[0232]A: less than 0.5%B: 0.5% or more and less than 1.0%C, 1.0% or more
and less than 2.0%D: 2.0% or more

[0233]<Image Uniformity (Density Variation)>

[0234]The 90h image was printed out on the entire area of three A3 sheets
of paper. The evaluation of the image was made on the third copy. For
evaluation of the image uniformity, the image densities at five locations
were measured and the difference between the maximum and the minimum was
measured. The image density was measured by an X-Rite color reflection
densitometer (color reflection densitometer X-Rite 404A).

A: less than 0.04B: 0.04 or more and less than 0.08C: 0.08 or more and
less than 0.12D: 0.12 or more

[0235]<Change of the Image Density by Allowing to Stand after the
Durability Test>

[0236]After the durability test at N/N and H/H, the FFh image (5
cm×5 cm) was printed out on 3 sheets of paper, and the image
density of the third copy was measured. The main body of the evaluation
apparatus was allowed to stand in each environmental condition for 3
days, and then the FFh image (5 cm×5 cm) was printed out on one
paper to measure the image density for evaluation of the density
difference before and after allowing to stand. The density was measured
by the above-mentioned color reflection densitometer X-Rite.

A: 0.00 or more and less than 0.05B: 0.05 or more and less than 0.10C:
0.10 or more and less than 0.20D: 0.20 or more

[0237]<Carrier Adhesion>

[0238]The carrier adhesion before and after the durability test at N/N was
evaluated. The 00h image was printed and a transparent adhesive tape was
contacted on the electrostatic image carrier (photoconductor drum) for
sampling. The number of magnetic carrier particles adhered on the
electrostatic image carrier (the area of 1 cm×1 cm) was counted to
calculate the number of adhered carrier particles per cm2.

A: 3 or lessB: from 4 to 10 (inclusive)C: from 11 to 20 (inclusive)D: 21
or more

[0239]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0240]This application claims the benefit of Japanese Patent Application
No. 2008-200644, filed Aug. 4, 2008, which is hereby incorporated by
reference herein in its entirety.